The proper securing of a parked vehicle, such as the trailer of a tractor-trailer, to a loading dock during loading and unloading operations is an essential safety precaution. Various vehicle restraints have previously been provided for this purpose, as disclosed, for example, in U.S. Pat. No. 4,208,161 (Hipp et al.), U.S. Pat. No. 4,264,259 (Hipp), U.S. Pat. No. 4,282,621 (Anthony), U.S. Pat. No. 4,373,847 (Hipp et al.), U.S. Pat. No. 4,379,354 (Hahn et al.), U.S. Pat. No. 4,443,150 (Hahn et al.), U.S. Pat. No. 4,472,099 (Hahn et al.), U.S. Pat. No. 4,560,315 (Hahn), and Re. 32,968 (Hahn).
One common type of vehicle restraint 40 is shown in FIGS. 1 and 2 of the drawings. This particular type of vehicle restraint 40 includes a carriage assembly 50 which is adjustably mounted within a track 60 for substantially vertical movement adjacent to a front wall 22 of a loading dock 20, a generally hook-shaped restraining member 70 which is housed within and pivotably attached to the carriage assembly 50, and a slope extension 80 which is pivotably mounted to the forward end of the carriage assembly 50 at pivot point 52. As is customary in the art, the carriage assembly 50 is biased by one or more resilient biasing elements (not shown) to assume an upper vertical position, as shown, for example, in FIG. 1, but may be moved downwardly into a lower vertical position, as shown, for example, in FIG. 2, when an external force is exerted on the vehicle restraint 40. By way of illustration, when the trailer 30 of a tractor-trailer moves backwardly towards the loading dock 20, engagement between an ICC bar or rear-impact guard 32 of the trailer 30 and either the slope extension 80 or an exposed cam surface 54 of the carriage assembly 50 causes an external force to be exerted on the vehicle restraint 40 which tends to move the carriage assembly 50 downwardly toward a vehicle supporting surface, such as roadway 24. The purpose of this upward biasing of the carriage assembly 50, and its downward positioning by the ICC bar 32, is to ensure that a proper orientation is obtained between the restraining member 70 and the ICC bar 32 before the restraining member 70 is moved into an engaging position, as shown in FIG. 2.
In order to avoid interference between the slope extension 80 and the roadway 24 as the carriage assembly 50 is being lowered, the slope extension 80 is pivotably mounted to the forward end of the carriage assembly 50 in a manner which permits the slope extension 80 to rotate upwardly out of the way. To this end, a stop member 56 is provided beneath the top portion of the slope extension 80 to permit the slope extension 80 to rotate about pivot point 52 in a clockwise direction, as indicated by reference numeral 82 in FIG. 1, but not in a counter-clockwise direction. More specifically, the stop member 56 allows the slope extension 80 to rotate from a normal pendent position wherein the slope extension 80 forms an extension of the exposed cam surface 54 of the carriage assembly 50, as shown in FIG. 1, to a raised position wherein the slope extension 80 and the exposed cam surface 54 form an angle of less than 180.degree..
While vehicle restraint 40 performs in a generally reliable manner, its carriage assembly 50, in some instances, may be obstructed from moving downwardly toward the roadway 24. In such instances, the carriage assembly 50 suffers from what is known in the art as a "dead lock" condition. One example of a "dead lock" condition occurs when the ICC bar 32 of a given trailer 30 is lower in elevation than pivot point 52 after the slope extension 80 contacts the roadway 24, as shown, for example, in FIG. 2. When this happens, the ICC bar 32 of the trailer 30 obstructs clockwise rotational movement of the slope extension 80, as viewed in FIG. 2. At the same time, engagement between the slope extension 80 and the roadway 24 obstructs further downward movement of the carriage assembly 50. As a result, the trailer 30 is obstructed from backing further toward the front wall 22 of the loading dock 20, thus inhibiting the intended securement of the trailer 30 to the loading dock 20.